The present invention relates to a method and apparatus for damping rocking mode vibrations in rotating devices. In particular, the present invention relates to a rocking mode vibration damper for computer disk drives.
Disk drive memory systems are used in personal computer applications to store digital information on magnetic disks. In a typical disk drive assembly, information is stored on the disks in concentric tracks divided into sectors. The disks themselves are mounted on a hub, which rotates relative to the disk drive enclosure. Information is accessed by means of read/write heads mounted on pivoting arms that move radially over the surface of the disks. This radial movement of the transducer heads allows different tracks to be accessed. The disks are rotated by an electric motor to allow the read/write head to access different sectors on the disks. The motor is typically mounted within or beneath the hub, and usually rotates the disks at from 1,200 to 12,000 revolutions per minute.
Vibrations in a disk drive occur as a result of several influences. Defects in the ball bearings interconnecting the rotating hub to the disk drive enclosure are a first cause of vibrations. Ball bearings are not perfectly spherical and generally they contain some defects such as flat spots, crevices, or the like. Similarly, ball bearing races may also contain defects. Consequently, the movement of the hub as the bearing passes each defect produces an excitation which generates vibration in the hub. Because there will be several bearing defect frequencies associated with each hub speed, a multitude of ball bearing excitation frequencies can result, producing vibrations in the disk drive assembly. Environmental vibrations or shocks are another cause of vibration in computer disk drives. Sources of environmentally-induced vibrations include physical jarring of the disk drive installed in a computer, or any movement of the computer enclosing the disk drive. Deformations of the disk drive enclosure or base plate, known as vertical diaphragm vibrations, are yet another cause of vibrations in computer disk drives.
Three modes of vibration may occur in a spinning disk and hub assembly. The first mode of vibration for the disk and hub is in a radial direction relative to the axis of rotation. The second mode of vibration for the disk and hub is in an axial direction relative to the axis of rotation. The third mode of vibration is a rocking displacement of the disk and hub relative to the axis of rotation. When these vibrations are of sufficient amplitude, they can cause servo system errors and transducer/track misregistrations, thereby decreasing or inhibiting drive performance.
Every rotating disk drive inherently has an upper and lower rocking mode vibration. This rocking mode vibration is the result of the manufacturing tolerances of the bearing itself and the structural stiffness of the spindle motor and the disk drive housing or enclosure. Thus, any disk drive hub will exhibit specific upper and lower rocking resonances, the frequency of which will change depending upon the number of disks supported by the spindle, as well as the rotational speed of the hub. The rocking resonance can be affected by defects in the disk drive assembly bearings. In particular, the frequency of the vibrations caused by the disk drive assembly bearings can resonate with the natural rocking mode frequency of the disk drive assembly and result in high amplitude vibrations. These high amplitude vibrations can cause data misregistration errors as the storage disks rock beneath the read/write head. In addition, such nonrepetitive runout, if it occurs during servo or data write operations, can later reduce the track following capability of the disk drive during read operations. This in turn can lead to read/write head servo system failure or an increase in acoustical noise. Therefore, reductions in rocking mode vibrations and/or bearing defect vibrations can reduce disk drive errors, can lead to longer read/write head servo system life, and can reduce the level of noise produced by the disk drive. Also, the amplitude of these vibrations is directly associated with drive performance. In drives where the amplitude is kept relatively small, data can be stored in higher densities than in drives where such vibrations have a relatively large amplitude.
The frequencies at which the rocking mode and bearing defect vibrations interact will vary, depending on the particular set of bearings used in the drive and the geometry of the drive itself. The number and thickness of storage media disks carried by a disk drive also affects the frequencies at which rocking mode vibrations occur. For example, where a disk drive structure is used to carry from one to four storage media disks and is used in a first family of drives operated at a first speed and a second family of drives operated at a second speed, there are 16 upper and lower dynamic rocking mode frequencies. In addition, there are two distinct sets of bearing defect frequencies. Therefore, it is virtually impossible to avoid all harmful frequency interactions using a single disk drive design for a family of multiple disk disk drives. As a result, disk drive manufacturers are compelled to use a wide variety of spindle motor designs to accommodate the various disk drive platforms and performance standards that comprise their product lines while simultaneously attempting to minimize the effects of vibration.
The structure of the disk drive also has a significant effect on the amplitude of vibrations resulting from rocking mode and vertical diaphragm mode resonant frequency vibrations. Undamped structures tend to exhibit vibrations having a higher amplitude at their resonant frequencies than do equivalent structures that contain damping means. Consequently, for a given vibrational input (e.g. from spindle bearing defects) an undamped and rigid disk drive assembly will produce larger amplitude vibrations in the disk storage media at resonant frequencies than will equivalent disk drives having damping means. Conversely, disk drive assemblies having damping typically exhibit resonant frequencies of a lower amplitude. Because of the relatively low amplitude of the vibrations in a damped drive, data storage densities may be increased compared to undamped drives.
The natural resonance of the disk drive assembly during normal operation is also a major source of acoustical noise. The reduction of acoustical noise is an increasingly important consideration in the design of computer disk drives. In particular, where the computer disk drive is for use in computers placed in work spaces, it is desirable that the disk drive have a very low acoustical noise signature. By reducing the amplitude of the resonant frequencies in a disk drive assembly, damping lowers the acoustical noise signature of the assembly.
Efforts to dampen vibrations in disk drive assemblies have included the insertion of damping material between the spindle and the drive enclosure. An example of such an approach is found in U.S. Pat. No. 5,483,397 to Gifford et al. However, this approach cannot be applied to disk drive designs having rotating shafts. Other designs have attempted to reduce vibrations by mounting the stator of the disk drive spindle motor to the base through a damping material. This approach is taken in U.S. Pat. No. 5,365,388 to Maughan et al. Although such a design is useful at damping vibrations occurring in the stator, it is incapable of damping rocking mode vibrations set up in the rotating spindle and hub. Also, such a design requires more space than an otherwise identical design not having damping material. This is a disadvantage because disk drives increasingly must provide greater storage capacity and smaller form factors.
In light of the above-described problems, including those of misregistration errors and acoustical noise, it would be useful to find a way to avoid or dampen vibrations generated due to natural rocking resonances and bearing defect frequencies in a disk drive assembly. Furthermore, it would be useful to shift the frequencies of the natural rocking modes to prevent the overlap of these rocking mode frequencies with the bearing defect frequencies at the operating speed of the disk drive assembly. In addition, it would be useful to provide a disk drive assembly that could be used over a range of disk drives, containing differing numbers of storage media disks. Furthermore, it would be advantageous to provide a disk drive assembly that is capable of accommodating variations in the rotational speed of the storage media and various types of bearings, and to avoid or dampen the effects of vibration overall.
In accordance with the present invention, a method and apparatus for damping vibrations in a computer disk drive is disclosed. More particularly, the present invention dampens vibrations in disk drive assemblies having a rotating shaft. In such devices, the rotating shaft interconnects the base of the disk drive assembly and the rotating hub. Typically, the hub and spindle rotate in unison with the rotating portions of a bearing assembly that is directly or indirectly affixed to the base plate of the drive. In a preferred embodiment, the bearing assembly is supported in the disk drive enclosure by damping material at a first portion, and by direct interconnection to the base at a second portion. The damping material thus may be interposed at an interface between the stationary portion of the bearing assembly, such as an outer race of a ball bearing, and the base of the disk drive apparatus.
The damping material interposed between the stationary base and the rotating spindle can be provided in a variety of forms. According to one embodiment of the present invention the damping material is an elastic O-ring. According to another embodiment of the present invention, the damping material is a damping adhesive. The precise composition of the damping material is relatively unimportant, so long as it is capable of absorbing and dissipating vibrational energy. Therefore, suitable materials include rubber, urethane, plastic, or any elastomeric or viscoelastic material.
According to a first embodiment, the disk drive assembly of the present invention includes a bearing assembly that is flexibly interconnected to the base assembly at a first portion, the flexible interconnection being through a damping material, and rigidly interconnected to the base at a second portion. In this way, large amplitude vibrations can be damped by the damping material, while maintaining a precise relationship between the spindle and the base assembly. According to this embodiment, the damping material is provided in the form of an O-ring. The rigid interconnection between the bearing assembly and the base is made at a level corresponding to the plane of the base underlying the stator assembly. The O-ring vibration damper of the flexible interconnection is generally located within a diameter described by a sleeve that extends from the base to receive the bearing assembly. Between the flexible interconnection and the rigid interconnection of the base sleeve and the bearing assembly is a gap that allows the bearing assembly to move relative to the base sleeve. Energy associated with this motion is absorbed by the damping material, thereby reducing the amplitude of the vibrations.
In another embodiment of the present invention, the base of the disk drive assembly has integral to it a sleeve to receive a bearing assembly. According to this embodiment, at a first end proximate to the base, the sleeve has a diameter greater than the bearing assembly. At a second end, distal from the base, the sleeve has a reduced inner diameter, corresponding to the diameter of the bearing assembly. Thus, when the bearing assembly is fully inserted in the base sleeve, a gap exists at the end of the sleeve corresponding to the base, while the end distal from the base is in closely fitting contact with the bearing assembly. Damping material is interposed in at least a portion of the gap between the base sleeve and the bearing assembly. According to this embodiment, the damping material may be provided in the form of an O-ring. Grooves may be provided in the base sleeve, the bearing housing, or both, to locate the damping material.
According to yet another embodiment of the present invention, a disk drive assembly is provided having a base sleeve with an increased interior diameter at a portion distal from the base. At a portion of the base sleeve interior to the base itself, the diameter of the base sleeve is reduced such that it can receive a bearing assembly in closely fitting contact. A layer of damping material is interposed within the gap formed between the distal portion of the base sleeve and the constant diameter bearing assembly. According to this embodiment, the damping material may be a damping adhesive.
A further embodiment of the present invention discloses a disk drive assembly having a base sleeve with a large diameter lower portion, adjacent to the base, and a reduced diameter upper portion. The base sleeve receives a bearing assembly or cartridge having a constant diameter approximately equal to the reduced diameter distal portion of the base sleeve. Upon insertion into the base sleeve, the bearing housing is in closely fitting contact with the distal portion of the base sleeve, and a gap exists between the bearing housing and the proximal portion of the base sleeve. Inserted into this gap is damping material, to absorb energy from vibrations between the base sleeve and the bearing housing. According to this embodiment, the damping material may be a damping adhesive placed on the large diameter interior of the base sleeve before the bearing housing is positioned in the sleeve.
The unique method and apparatus for reducing vibration in a disk drive assembly provided by the present invention has a number of advantages. According to the present invention, while vibrational energy is absorbed and the amplitude of vibrations therefore reduced, the positioning of the components relative to one another can be maintained with high precision. Furthermore, by absorbing vibrational energy, the acoustical noise of the disk drive assembly is reduced. The present invention also allows the density of data tracks on disk storage media to be increased as a result of the decreased amplitude of drive vibrations. Also, the present invention provides a disk drive assembly having a rotating shaft design in which vibrational energy is dampened.
The present invention allows for the damping of vibrations in a computer disk drive assembly, even where a rotating shaft is utilized. Additional advantages of the present invention will become readily apparent from the following discussion, particularly when taken together with the accompanying drawings.